Integrated heater element array and drive matrix and method of making same

ABSTRACT

Thermal display including an air isolated integrated semiconductor circuit forming a semiconductor heater element array joined by a metallic connecting pattern which extends out over the heating elements to interconnect selected ones of them and a PN junction isolated integrated semiconductor drive matrix for the heating element array positioned in the same plane as the heating element array. The PN junction isolated integrated semiconductor drive matrix and the semiconductor heating element array are concurrently formed in the same semiconductor substrate and the heating element element array is air isolated to provide a high degree of electrical and thermal isolation for the heating element array while both are located in the same plane on a larger support. The thermally sensitive material on which a dynamic display is formed or on which a permanent display is printed is in direct contact with the monocrystalline semiconductor material of the heating circuit array and can be passed over the heating element array and the drive matrix.

United [72] Inventors States Patent Jerry D. Merryman; Edward M. Ruggiero, Dallas, Tex.

1211 Appl. No. 847,751

{22] Filed [73] Assignee Texas Instruments incorporated Dallas, Tex.

[54] INTEGRATED HEATER ELEMENT ARRAY AND DRIVE MATRIX AND METHOD OF MAKING SAME which extends out over the heating elements to interconnect 4Claims, 5 Drawing Figs selected ones of them and a PN junction isolated integrated semiconductor dr1ve matrix for the heating element array [52] US. Cl 29/573, iti n d in the ame plane as the heating element array The 29/577 PN junction isolated integrated semiconductor drive matrix Int. d the emi onductor heating element array are concur. HQlU rently formed in the same semiconductor substrate and the [50] Field Of Search ..-29/573, 569 l a ma ay id a high 577, 580, 590 egree of electrical and thermal isolation ior the heatingele- [56] R f Cted ment array while both are located in the same plane on a e erences I larger support. The thermally sensitive material on which a UNITED STATES PATENTS dynamic display is formed or on which a permanent display is 2,922,993 1/1960 Sack 29/569UX printed is in direct contact with the monocrystalline semicon- 3,l22,680 2/ 1964 Benn 29/577UX ductor material of the heating circuit array and can be passed 3,323,241 6/1967 Blair et al. 40/28 over the heating element array and the drive matrix.

2 3 l l r f -14 l BUD ---7 BUD l r'-'---\ EIDEl l L JT "6 5E5 l MEsAs 3,354,817 11/1967 Sakurietal ABSTRACT: Thermal'display including an air isolated integrated semiconductor circuit forming a semiconductor heater element array joined by a metallic connecting pattern R PATENTED was I971 SHEET 1 OF 3 n v ..m mm En T RI l I MEsAs 3 nwsmox JERRY 0. MERRYIMN EDWARD M. RU66IERO ATTORNEY PATE NT-Eumaa lsn SHEET '2 OF 3 lNTEGlhATElD HEATER ELEMENT ARRAY AND DRIVE lVlATmli ANT) MEET-3010GT MAKQING SAME This is a division of application Ser. No. 671,821. filed Sept. 29, i967, now US. Pat. No. 3,501 ,615.

The present invention relates to thermal displays of the type having an array of heater elements selectively energized to provide an information display on thermally sensitive material and more particularly to an integrated semiconductor heater element array and drive matrix therefore and to methods of making them.

An object of the present invention is to provide an improved and simpler thermal display.

An object of the present invention is to provide an integrated semiconductor circuit tailored to meet different electrical and thermal requirements useful for a thermal display.

Still another object of the present invention is to provide an improved and simpler method of fabricating an integrated semiconductor circuit useful for a thermal display.

Other objects, features, and advantages of the invention may be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings in which like reference numerals indicate like parts and in which:

FIG. 1 illustrates an integrated semiconductor heater element array and drive matrix according to the invention;

HQ. 2 illustrates an intermediate structure in the fabrication of integrated semiconductor heater element array and drive matrix of HG. ll;

lFlG. 3 illustrates the interconnection pattern of the heater elements and drive matrix on the surface of the structure of H6. 2;

H6. i illustrates the interconnection pattern for external connection to the heater elements and drive matrix of FIG. 11;

MG. 5 illustrates the electrical circuit embodied in the integrated heater element array and drive matrix of FIG. 1.

FllG. it illustrates a 3 by 5 heater element array of semiconductor means located within the window 3 and the drive matrix 4i over which thermally sensitive material is positioned to form a dynamic information display of the type described in U.S. Pat. No. 3,323,241 by J. W. Blair et al. in which the described thermochromic materials are used or over which is passed a specially treated thermally sensitive material to form a permanent information display or printer of the type described in copending application Ser. No. 492,174 by Emmons et al., filed Oct. 1, 1965, and assigned to the assignee of the present application.

A monocrystalline silicon semiconductor wafer 2 is mounted on a larger insulating support l which may be any suitable material, for example, ceramic, glass or sapphire, by way of an insulating adhesive having good thermal and electrical insulating properties such as epoxy.

Each heater element of the array comprises a monocrystalline semiconductor body in a mesa shape and contains a heater element formed therein at the underside of the mesa adjacent the support 1 so that when the heater element is energized, a "hotspot is formed at the top surface of the mesa to provide a localized dot on the thermally sensitive material above it. A group of selectively energized heater elements forms a group of dots on the thermally sensitive material defining a character or information representation displayed on the thermally sensitive material.

The mesas comprising the heater element array are air isolated from each other and joined by a metallic connecting pattern underneath the mesas between the semiconductor wafer 2 and the support l which pattern interconnects the heater elements in the mesas in the desired circuit configuration. The drive matrix for selectively energizing the heater elements and supplying the desired power to the heater elements is located in the semiconductor wafer 2 in the area generally designated as i. The circuit elements forming the drive matrix are integral within the semiconductor wafer 2, PN junction isolated from array and the drive matrix are also interconnected in the desired circuit configuration by the metallic connecting pattern between the wafer 2. and the support ll.

one another and interconnected in the desired circuit configu- Y ration by a metallic connecting pattern underneath the wafer The semiconductor wafer 2 is integral except within the window 3 in which are located the air isolated heater elements and consequently the top surface of the semiconductor wafer 22 presents a good, more uniform support for the positioning or passing of the thermally sensitive material over the heater ele ment array.

The metallic connecting pattern located between the semiconductor wafer 2 and the support i extends out into bonding pads located above the openings 5, 6 and 7 in the support 31 so that external connection can be made to these bonding pads through the openings at the underside of support 11. Whereas, the external connections are formed at the underside of support ii and are removed from the thermally sensitive material located above the mesas. The metallic connecting pattern located between the semiconductor wafer 2 and the support it mechanically and electrically joins the air isolated mesas and electrically connects them to the circuit elements of the drive matrix and is supported in the epoxy adhesive resting between the semiconductor wafer 2 and the support l.

Each mesa contains a transistor-resistor pair which is selectively energized so that the power dissipated by the resistor causes the hotspot" at the top surface of the selected mesa. The transistor in each mesa provides an active control or amplifying function in the manner that the heat generated by it facilitates the creation of the hotspot. Moreover, an active element in each mesa lessens the need for amplification of signals that would otherwise have who provided externally to the heating element array and allows the heating element array to operate directly from low power driving sources.

The transistor-resistor pair in each mesa is illustrated in MG. 5, transistor TM and resistor RM for example along with its associated drive circuitry, transistor T2@, resistor R ZEi, resistor R 29 and resistor R E?) for example. Each transistor-resistor pair is interconnected in the manner that one end of the resistor is connected to the collector of the transistor, the other end of the resistor being connected to a positive voltage source V the emitter of the transistor being connected to ground and the base of the transistor being connected to the drive circuit (i.e. the emitter of the associated transistor in the drive circuit).

Upon the simultaneous application of positive pulses at the input terminal T29 and the terminal PG, the transistor T29 is turned on, causing the voltage at the emitter of transistor T29 to become more positive and trigger the transistor Tlld causing the hotspot at the surface of the mesa in which the transistor TM and resistor R14 are located. The line P6 is connected to all the transistors T29, Tlill through the resistors R 29, R 30 in the manner that the simultaneous appearance of a positive pulse at PG and a selected one of the inputs 129 or llih causes the selected transistor T29 or Tlili to turn on and in turn trigger the selected heating element.

in the example given, a 3 by 5 heating element array, there are 15 mesas, a corresponding 15 transistor-resistor pairs (TM-R14, T15-Rl5), a corresponding 15 drive transistors (T29, TM) and a corresponding 15 inputs (lZQ, I30).

The construction of the heater element array and the drive matrix of F 10. i may be better understood from the process of fabricating it.

Referring to FIG. 2, there is illustrated an integral monocrystalline semiconductor wafer 2 of P-type silicon. The transistor-resistor pairs for the heating elements comprise diffused regions in the surface of the wafer 2 and are illustrated as Tl through T15 and respectively Rl through R15 located in the area designated 3. 8 illustrates the area which is to be a mesa shape. Whereas, each transistor TllS for example comprises a diffused N-type collector region 9, a diffused l-type base region lid, and a diffused N-type emitter region lllr. its sistor R15 for example comprises a diffused N-.ype region made at the same time as the N-type collector diffusion and is 2 between the wafer 2 and the support l. The heating element integral therewith so that one end of the resistor if is ohmidugtiye tunnel in the cally connected to the collector 9 internally of the semiconductor material.

The drive transistors Tl6-T30 each comprise an N-type difiused collector region, P-type diffused base region and an N-type diffused emitter region. Each drive transistor T16- T30 has associated therewith a collector resistor respectively R m-R 30. The collector resistors R 16R 30 each comprise an N-type diffused region made at the same time as the respective collector diffusion of the drive transistor in the manner that one end of the collector resistor is integral with the collector of its associated drive transistor. Whereas, one end of the collector resistors R 16R 30 are respectively connected internally of the semiconductor material to the collectors of the drive transistors Tl6-T30. The diffused resistors R 2lR 25 have one end internally connected in the semiconductor material respectively to one end of the diffused resistors R 30, R 29, R 28, R 27 and R 26. The base resistors R 1630 are diffused P-type regions in the surface of the semiconductor wafer 2. These base resistors are to be connected to the base electrodes of the respective drive transistors T16-T30. The emitter resistors R l630 are diffused P-type regions in the surface of semiconductor wafer 2 and are to be connected to the emitter electrodes of the respective drive transistors T16-T30. A diffused N-type region in the surface of the semiconductor wafer surrounds each of the P-type diffused regions comprising the base and emitter resistors 11 16-30 and R 16-30 in order to provide the desired PN junction isolation between the circuit elements in the semiconductor material. Heavily doped N-type regions TJLI: T et s sonfiustfiiwwshin. the q ic aductor wafer 2 for providing ohmic electrical connection between the base electrodes of the respective transistors T l- T15 and the various circuit elements in the drive matrix. A he il y doped N-type diffused region T -C provides a con- W. -.miq liit rmaterial, T'shr fi doped N-type diffused regions PG are provided 151E surface of the semiconductor wafer 2 respectively near the three groups of resistors R 16-20-R 16-20, R 2125 R M-25 and R ,26-30R 2630. The P-N junction formed between an N-type tunnel and the subjacent P-type substrate isolates the tunnels from each other and from the ot h er circuit elements.

The transistors, resistors, tunnels and isolating junctions are formed in the surface of wafer 2 utilizing the planar process in which an oxide film is thermally grown on the P-type silicon wafer of the desired resistivity by placing it in a furnace at an elevated temperature and passing an oxidizing agent over it. The resulting silicon dioxide film acts as a masking medium against the impurities which are later diffused into the wafer. Holes are produced in the oxide film to allow subsequent diffusion processes to form the transistor, resistor, tunnel and isolating functions. These holes which are patterns of the desired circuit elements, tunnels and isolating regions are produced by photolithographic techniques. Contacts and interconnections between the circuit elements are made by similar photolithographic techniques using, for example, evaporated aluminum over the oxide to form a metallic pattern connecting the circuit elements together and terminating in bonding pads for external connections. The connecting pattern comprises conductive strips on the oxide film extending into openings in the oxide film for providing the desired connections and can be formed in the manner described in copending application Ser. No. 645,539 filed Jun. 5, 1967, entitled Method of Making Semiconductor Devices by Jack S. Kilby which is assigned to the assignee of the present application.

The metallic connecting pattern formed on the oxide on the semiconductor wafer 2 is illustrated in FIG. 3. A large conductive ground plane, designed ground in FIG. 3 interconnects all the emitters of transistors Tl-T15 and interconnects one end of all of the emitter resistors R M-30. R 20, R 25 and R 30 are illustrated in FIG. 3 to show the place where the ground plane connects to these emitter resistors. The conductive strip V interconnects one end of all the resistors R1- R15 and one end of the collector resistors R 1620. The conductive strip V' interconnects the common tenninals of the collector resistors R 21R 30 (designated V in FIG. 2) and one end of the tunnel Tye (designated V in FIG. 2). Conductive strip 36 connects the base of transistor T15 to one end of the tunnel T 15 and conductive strip 37 connects the other end of the tunnel T 15 to the emitter of transistor T30 and to one end of the emitter resistor R 30. The conductive strip 38 connects the base of transistor T14 to one end of the tunnel T 14 and conductive strip 39 connects the other end of the tunnel T 14 to the emitter of transistor 29 and to one end of emitter resistor R 29. In a like manner, the bases of all the transistors Tl-Tl5 are connected by way of the tunnels T ll5 to the emitters of transistors T16-30 and the emitter resistors R l630. Conductive strips 21-35 respectively connect to the bases of transistors 30, 29, 28, 27, 26, 21, 22, 23, 24, 25, 16, 17, 18, 19 and 20 and to one end of their base resistors. The enlarged portions of 2l-35 will later act as bonding pads for external connection and more specifically the inputs to selectively energize the heater elements. Whereas, the bonding pad 21 of FIG. 3 corresponds to the input of FIG. 5 and the bonding pad 22 of FIG. 3 corresponds to the input 129 of FIG. 5.

The other ends of the base resistors R M-30 are connected to the tunnels PG illustrated in FIG. 2 and the ends of these tunnels are interconnected by the conductive strip PG in FIG. 3. For example, the base resistor R 20 has its other end connected to the tunnel PG at the top of FIG. 2 by way of the conductive strip 41 illustrated in FIG. 3, the base resistor R 30 has its other end connected to the tunnel PG illustrated in the middle of FIG. 2 by way of the conductive strip 40 illustrated in FIG. 3 and the base resistor R 26 has its other end connected to the tunnel PG illustrated at the bottom of FIG. 2 by way of the conductive strip PG illustrated in FIG. 3.

It should be mentioned that where a conductive strip crosses over a tunnel, for example, the conductive strip V crossing over the tunnels T 1T,,l0, the silicon oxide insulating layer on the surface of the semiconductor wafer insulates the conductive strip from the conductive tunnel so that there is no electrical interference.

Accordingly, the drive matrix being more complex and requiring more circuit elements than the heating element array occupies an area of the semiconductor wafer larger than that of the heating element array and is near the heating element array while the two are fabricated during the same process operations and subjected to the same environments. The need for external driving circuitry is eliminated and the connecting pathway reduced.

After the semiconductor wafer is processed and includes the heater element array and the drive matrix with the desired connecting pattern as illustrated in FIG. 3, the wafer is turned upside down and mounted on a larger insulating support I in accordance with the procedure described in copending application Ser. No. 650,821 by Edward M. Ruggiero, filed Jul. 3, 1967, entitled Thermal Displays using Air Isolated Integrated Circuits and Methods of Making Same and assigned to the assignee of the present application. Whereas, a parting agent comprising photoresist material is selectively applied over the bonding pad areas designated by points 21-35, PG, R 30, V and G in FIG. 3. An epoxy adhesive is then applied over the semiconductor wafer on the metallic connecting pattern the silicon oxide and photoresist material. The epoxy adhesive adheres to the silicon oxide and the metallic connecting pattern but does not adhere to the photoresist material. The semiconductor wafer is then turned upside down and mounted on the insulating support 1 as illustrated in FIG. 1 with the bonding pads 31-35, V and G overlying the opening 5, the bonding pads 26-30 and V overlying the opening 6 and the bonding pads 21-25, R 30 and PG overlying the opening 7. These bonding pads are aligned with the openings 5-7 in such a manner that they will be accessible through the openings in the support.

Flt 4 illustrates the bottom view of the support 1 showing the openings 5-7 with the appropriate bonding pads located above the openings.

The epoxy adhesive is then cured into a rigid solid and during the initial curing process, the viscosity of the epoxy adhesive decreases considerably prior to polymerization and hardening. This lower viscosity of the adhesive facilitates flowing of the epoxy adhesive which will not readily wet the photoresist material thereby causing the epoxy adhesive to pull away from the photoresist material and collect in the areas around the photoresist material forming a meniscous with the wall of the openings 5-7 in the support 1.

After complete curing of the epoxy adhesive, the photoresist material is removed by conventional techniques leaving the bonding pads free from the epoxy adhesive and clean for making good electrical connections thereto.

The top surface of the semiconductor wafer which is the surface remote from the heater elements and the drive matrix elements is removed to make the semiconductor wafer as thin as desirable. This may be accomplished in one step or in multiple steps using lapping, sand blasting, or chemical etching.

However, the integrity of the PN junctions is maintained.

Since the thermally sensitive material will be positioned on or passed over the monocrystalline surface of the semiconductor wafer, it is chemically or mechanically polished.

The semiconductor material of wafer 2 around each transistor-resistor pair of a heater element is now removed to leave the 3X5 array of air isolated mesas. A photoresist layer is applied over the top surface of the wafer 2 and a photomask is applied over this photoresist layer to provide the desired exposure pattern for the photoresist layer. The photoresist layer is then exposed through the photomask, developed and selectively removed to leave exposed those areas of the semiconductor surface which are to be removed. With the photoresist layer defining the desired pattern, the semiconductor material is etched down to the silicon oxide film to leave the air isolated mesa shapes as illustrated in FIG. 1.

FIG. 1 illustrates the resulting shape of the semiconductor wafer 2 wherein is located the 3X5 array of air isolated mesas.

Referring now to FIG. 4 and looking at the underside of the insulating support ll, a metallic pattern previously applied on the underside of the insulating support 1 is to be connected with the bonding pads on the semiconductor wafer. Connections 42 are bonded between the bonding pads and the conductive strips on the underside of the insulating support 1 through the openings 57 in the insulating support.

As can be seen, the terminal strips 2l-35 in conjunction with terminal strip PG provides the input terminals for selectively energizing the heating element array'which was previously discussed in connection with input terminals l l and PG of HG. 5.'The power supply terminals are provided by strips V and G to provide the ground and collector voltage connections to the system.

The thermally sensitive material for display purposes is placed in direct contact with the monocrystalline silicon I mesas which are very thin thereby allowing a high degree of thermal communication between the mesas and the thermally sensitive material. The heating element array has a high degree of electrical and thennal isolation between the mesas and is particularly suitable for thermal display applications while a high density of circuit elements constituting the drive matrix may be integrated therewith with adequate electrical and thermal isolation.

The 5X3 array of mesas is given herein as an example since any number and shape of the array may be chosen depending upon the character of the information desired to be displayed on the thermally sensitive material.

it is to be understood that the above-described embodiment is merely illustrative of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

l. A method of making an integrated semiconductor circuit comprising the steps of: concurrently forming a plurality of heat dissipative elements at a first area of one face of a semiconductor wafer and forming at least as large a plurality of PN junction isolated circuit elements at a second spaced area of said one face of said semiconductor wafer, forming a conductive pattern over said one face of said semiconductor wafer interconnecting said heat dissipative elements and said plurality of PN junction isolated circuit elements, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from said semiconductor wafer in said first area leaving physically separated wafer parts each containing oneof said heat dissipative elements while maintaining said second area of said semiconductor wafer integral throughout. 2. A method of making an integrated semiconductor circuit, comprising the steps of: concurrently forming a plurality of heat dissipative elements at a first area of one face of a semiconductor wafer and forming a larger plurality of PN junction isolated circuit elements at a spaced larger second area of said one face of said semiconductor wafer, forming a conductive pattern over said one face of said semiconductor wafer interconnecting said heat dissipative elements and said plurality of PN junction isolated circuit elements, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from the opposite face of said semiconductor wafer in said first area leaving physically separated wafer parts each containing one of said heat dissipative elements while maintaining said second area of said semiconductor wafer integral throughout.

3. A method of making an integrated semiconductor circuit, comprising thesteps of: concurrently forming a plurality of diffused heat dissipative elements at a first area of one face of a semiconductor wafer and forming a larger plurality of diffused PN junction isolated circuit elements at a larger spaced second area of said one face of said semiconductor wafer, forming a conductive pattern on insulating material on said one face of said semiconductor wafer interconnecting said diffused heat dissipative elements and said plurality of diffused PN junction isolated circuit elements, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate. and selectively removing semiconductor material from the opposite face of said semiconductor wafer in said first area leaving physically separated wafer'parts each containing one of said heat dissipative elements while maintaining said second area of said semiconductor wafer integral throughout.

4. A method of making an integrated semiconductor circuit comprising the steps of: concurrently forming a plurality of diffused resistors at a first area of one face of a semiconductor wafer and a like plurality of PN junction isolated transistors at a larger spaced second area of said one face of said semiconductor wafer, forming a conductive pattern over said one face of said semiconductor ,wafer interconnecting said diffused resistors and said transistors, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from said semiconductor wafer in said first area leaving physically separated wafer parts each containing one of said diffused resistors while maintaining said second area of said semiconductor wafer integral throughout. 

2. A method of making an integrated semiconductor circuit, comprising the steps of: concurrently forming a plurality of heat dissipative elements at a first area of one face of a semiconductor wafer and forming a larger plurality of PN junction isolated circuit elements at a spaced larger second area of said one face of said semiconductor wafer, forming a conductive pattern over said one face of said semiconductor wafer interconnecting said heat dissipative elements and said plurality of PN junction isolated circuit elements, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from the opposite face of said semiconductor wafer in said first area leaving physically separated wafer parts each containing one of said heat dissipative elements while maintaining said second area of said semiconductor wafer integral throughout.
 3. A method of making an integrated semiconductor circuit, comprising the steps of: concurrently forming a plurality of diffused heat dissipative elements at a first area of one face of a semiconductor wafer and forming a larger plurality of diffused PN junction isolated circuit elements at a larger spaced second area of said one face of said semiconductor wafer, forming a conductive pattern on insulating material on said one face of said semiconductor wafer interconnecting said diffused heat dissipative elements and said plurality of diffused PN junction isolated circuit elements, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from the opposite face of said semiconductor wafer in said first area leaving physically separated wafer parts each containing one of said heat dissipative elements while maintaining said second area of said semiconductor wafer integral throughout.
 4. A method of making an integrated semiconductor circuit comprising the steps of: concurrently forming a plurality of diffused resistors at a first area of one face of a semiconductor wafer and a like plurality of PN junction isolated transistors at a larger spaced second area of said one face of said semiconductor wafer, forming a conductive pattern over said one face of said semiconductor wafer interconnecting said diffused resistors and said transistors, mounting said semiconductor wafer on an insulating substrate with said one face of said semiconductor wafer adjacent said insulating substrate, and selectively removing semiconductor material from said semiconductor wafer in said first area leaving physically separated wafer parts each containing one of said diffused resistors while maintaining said second area of said semiconductor wafer integral throughout. 